In the editor you can resize place holders to whatever size you'd like by dragging the arrows in the corners of the place holder. You can see this in the image below. You can make the place holder bigger by dragging those dots that are in the corners/edges.
This could also an issue with the display resolution on the presentation. This likely isn't your issue here as you can see the blue background stretching across the whole screen. You cant try pressing the settings (cog wheel) icon that is near the name of your presentation. Form there you can change the resolution of your presentation. It looks like it may need increased